Working code is worth its weight in gold.

Write your tests.
Write your code.
Update your diagrams.
Done.

See the mention of copious reams of documentation here?
No?
That's because there really is no need to write them.
Why?
Because there is no point in spending hours of programmer time trying to write a document that will never be read.

Class diagrams are useful to quickly illustrate the architecture to newcomers but that's all the additional documentation you should need to explain how things work.

If your code is written well - i.e. well laid out classes with suitably short, well-named functions that do just one thing - the tests and the working code will be sufficient to easily explain what it does and how it does it.

It should be like looking up the word "dictionary" in a dictionary. It's functional - in giving the definition - and it also explains what it is.